DC to DC converters are important in portable electronic devices such as cellular phones and laptop computers, which are supplied with power from batteries. Such electronic devices often contain several sub-circuits which each require unique voltage levels different than supplied by the battery (sometimes higher or lower than the battery voltage, or even negative voltage). Additionally, the battery voltage declines as its stored power is drained. DC to DC converters offer a method of generating multiple controlled voltages from a single variable battery voltage, thereby saving space instead of using multiple batteries to supply different parts of the device.
Electronic switch-mode DC to DC converters are available to convert one DC voltage level to another. These circuits, very similar to a switched-mode power supply, generally perform the conversion by applying a DC voltage across an inductor or transformer for a period of time (usually in the 100 kHz to 5 MHz range) which causes current to flow through it and store energy magnetically, then switching this voltage off and causing the stored energy to be transferred to the voltage output in a controlled manner. By adjusting the ratio of on/off time, the output voltage can be regulated even as the current demand changes. This conversion method is more power efficient (often 80% to 95%) than linear voltage conversion which must dissipate unwanted power. This efficiency is beneficial to increasing the running time of battery operated devices. A drawback to switching converters is the electronic noise they generate at high frequencies, which must sometimes be filtered.
There are several known techniques for controlling the switching device(s) of a switched-mode converter. In conventional current mode DC-DC converters, the duty cycle of the switching device of the converter is modulated by a negative feedback voltage loop to maintain the desired output voltage. The negative feedback loop ordinarily includes a voltage error amplifier that compares a signal indicative of the output voltage to a reference voltage. In typical current mode control circuits, when the sum of the sensed transformer current and the compensating ramp from the voltage error amplifier exceed an error current signal, a latch is reset and the switching device is turned OFF.
To enhance power-processing density, or to meet increased current demands of the load, it is often desirable to connect several switch-mode converters in parallel. In a typical paralleled switch-mode converter configuration, each converter is designed to contribute an equal amount of current to the load in the presence of inevitable variations in reference voltages and component values. Several techniques for enhancing current sharing between paralleled converters are known, as is evidenced in the following U.S. Pat. No. 6,768,658 (DC-DC power supply with at least two paralleled converters and current share method for same); U.S. Pat. No. 6,137,274 (Switching DC-to-DC converter and conversion method with current sharing between paralleled channels); U.S. Pat. No. 5,838,151 (Wireless load sharing for parallel power converters and method); U.S. Pat. No. 5,740,023 (Control system for a modular power supply and method of operation thereof); U.S. Pat. No. 5,164,890 (Current share scheme for parallel operation of power conditioners); U.S. Pat. No. 5,036,452 (Current sharing control with limited output voltage range for paralleled power converters); U.S. Pat. No. 4,635,178 (Paralleled DC power supplies sharing loads equally); and U.S. Pat. No. 4,149,233 (Circuit for automatic load sharing in parallel converter modules).
One control technique is to derive a share function that uses a sensor amplifier to generate a share bus that is proportional to the total load current. A slow speed servo loop can then adjust the voltage loop of each converter to force balance of the load current between the paralleled converters.
One state of the art load sharing controller is the LTC4350 from Linear Technology Corporation of Milpitas, Calif. The datasheet for this device is attached to the Provisional Application to which this application claims priority as an Appendix, and a typical application of the LTC4350 controller is provided in FIG. 1. The LTC4350 is a load share controller that allows systems to equally load multiple power supplies connected in parallel. The output voltage of each supply is adjusted using the SENSE+ input until all currents match the share bus. This concept is typical for the control of commercial DC-DC converters or AC-DC converters that are marketed today. They generally have an extra signal connected between each of the supplies that will source the shared load. The connection of the Linear Tech LTC4350 can be seen in as the “Share Bus” FIG. 1.
A further, simpler, known control scheme for controlling the output converters provided in parallel can be described with respect to FIG. 2, which illustrates an LTC3782 DC-DC converter from Linear Technology Corporation (a data sheet for this device is also included in the Appendix to the Provisional Application from which this application claims priority), and, in particular, a feedback voltage divider network used with this device. The output voltage of the circuit is set based on the input of this voltage divider to a feedback pin on the converter—the ratio of R1/R2 determines the output voltage of the circuit.
In addition, this device includes a feature, common to many converter circuits, that lowers the voltage when the output current is at its maximum allowable value. This “fold back” protects the circuits by lowering the output voltage, and in turn hoping the output current lowers to an allowable level.
When a conventional pair of converter circuits, such as two LTC3782s, are connected in parallel to load share, one device will typically source all of the current output from the pair. This is due to slight differences in the output voltage between the circuits and the high gain error amplifiers that help control the output voltage. The circuit with the slightly higher output voltage will supply the power. When the regulator supplying the current reaches its current limit, it starts to fold back the output voltage. When the output voltage lowers enough to match the output voltage of the second circuit sharing the load, the second circuit will start increasing its current as the load current demand increases. This crude form of current sharing will force the circuit with the slightly higher output voltage to do most of the work.
While this control scheme is considered a crude one for a number of reasons, it does have the advantages of being relatively inexpensive to implement and it does not require additional wiring between the converters. If load share controller circuits were added, such as the LTC4350 shown in FIG. 1, these circuits would current share with the addition of an extra wire and all the required circuits.
One object of the present invention is to provide better load sharing results without adding extra wires or connections.